This invention relates generally to the deposition of silver films on nonconducting substrates, and, more particularly, to deposition of such films that are very thin.
Thin films of certain metals on nonconducting substrates can have important commercial applications. Thin films of conducting metals on transparent substrates are used in electronic display devices. Thin films can be used to reflect heat in solar shading or other solar devices, and to filter radiation from sunlight. A thin film can reduce the incidence of infection caused by a device that is introduced into the human body, when the film is coated onto the device before introduction into the body. Thin films are used in packaging as a vapor barrier coating. These applications are only illustrative of the thousands of uses of thin films, and are not limiting of their uses.
In one particular application, films of silver or silver-containing compounds are particularly effective in reducing microbial infection in the human body. U.S. Pat. No. 4,404,197 describes the use of silver-containing salts in reducing the likelihood of infection of burn victims. U.S. Pat. No. 4,581,028 describes the use of antimicrobial silver salts in implants, and U.S. Pat. No. 4,603,152 describes other such devices utilizing silver compounds to resist infection. U.S. Pat. Nos. 4,054,139 and 4,483,688 disclose the use of a pure silver metallic coating on medical devices to reduce the incidence of infection. Thus, it is well established that coatings of silver or silver compounds are effective in reducing the chances of infection caused by medical devices that are implanted or inserted into the body.
Although the value of using silver to avoid infection is well established, there is less knowledge as to effective approaches to the best approach to providing the silver on the surface. Electrodeposition might be used, but in most cases the medical instruments are made of nonconducting materials which cannot be readily coated electrolytically on a commercial scale, with a thin, adherent coating. The '139 patent suggests that coating "of the type deposited by electroless deposition" would be operable, but gives no details of operable processes. The '688 patent describes the use of large 300 mesh particulate silver to coat catheters.
Silver can be coated onto nonconducting substrates by electroless processes. One example is the process used to coat silver onto mirrors, but such coatings are comparatively thick. Another example is dry deposition techniques such as vapor deposition or sputtering, but these cannot be used to coat irregularly shaped substrates, or the insides of long bores.
Because silver can be toxic in some circumstances, and is expensive, it is preferable to coat the silver as a very fine layer onto the electrically nonconducting substrate. The coating should be strongly adherent to the substrate, because loss of the coating might result in infection or passage of the silver into the body. There are not presently known techniques for depositing silver onto various types of nonconducting substrates that permit the deposition of a very thin, but uniform, transparent layer of silver, on the order of 2 to 2000 Angstroms thick, produce a highly adherent layer with good mechanical properties, and are readily adapted to large scale commercial manufacturing of coated products.
There therefore exists a need for such a coating technology. The present invention fulfills this need, and further provides related advantages.